Gas is stored by compression and containment, by reversible adsorption into suitable storage materials, and/or by reversible chemical reactions that produce molecular compounds in one reaction direction and liberate gas in another direction. Gas separation is based typically on temperature swing adsorption, pressure swing adsorption, and/or chemical or physical scrubbing. These techniques may also be used for facilitating gas storage or concentration of a gas.
Temperature swing adsorption involves exposing a feed gas mixture (gases A and B) to an adsorbent bed. If gas “A” is selectively or preferentially adsorbed on the bed, the result is a gas blend enriched in gas B and depleted in gas A. When the adsorbent bed reaches full capacity for gas A, the feed gas mixture of gases A and B is diverted and the adsorbent bed is sealed off and heated, which liberates the adsorbed gas that is enriched in gas A. After liberating the adsorbed gas, the bed is cooled down and exposed again to the feed gas mixture. In a large scale industrial application, two or more adsorbent beds are typically used in tandem, leading to a continuous separation process. For an even more complete separation, beds can be staged in series such that gas liberated from one bed is the feed gas for another bed. Electric swing adsorption is a form of temperature swing adsorption where electricity is used to resistively heat the adsorbent bed to induce the thermal release of adsorbed material.
Pressure swing adsorption involves exposing an adsorbent bed to a high pressure gas to promote selective capture of one or more components of the gas by the bed. The bed can be regenerated by reducing the pressure to induce gas desorption from the bed.
Selective gas separation by chemical or physical scrubbing involves preferential uptake of one or more component(s) of a gas stream into a solution that functions as an adsorbent. The solution may rely on physical interactions with the gas, such as solubility, and/or may rely on chemical reaction between a gas and the solution. The gas-loaded solution is then typically heated (sometimes with a lowering of the pressure) to reverse the adsorption process and yield a gas enriched in specific components.
Gas separations and gas concentration are also performed by selective condensation of heavier components by compression and cooling. Membranes and clathrate inclusion compounds have also been used for gas separation.
Adsorbents used in gas separation methods based on physical adsorption, such as temperature and pressure swing adsorption, are varied. Adsorbents include activated charcoal, graphite, carbon nanotubes, and other allotropes of carbon. Natural and synthetically-modified minerals (e.g. zeolites or silica), polymers, and/or gels can also be used as absorbents.
More recently, Metal-Organic Frameworks (MOFs) which are porous and have a high surface area (up to 5000 m2/g) have been employed as adsorbents. Much of the interest in MOFs is due to their ability to stabilize hydrogen (“H2”) at liquid nitrogen temperatures. For example, one well-studied MOF structure known in the art as MOF-5 (see: Li et al., “Design and Synthesis of an Exceptionally Stable and Highly Porous Metal-Organic Framework,” Nature, volume 402, November 1999, pp. 276-279, incorporated by reference herein) can store up to 11 weight percent H2 at sub-atmospheric pressure at 30 K. Hydrogen is of considerable interest due to its potential use as a clean fuel for electric power production and in fuel cells for automotive applications. However, separation and storage of H2 remains a challenge due at least in part to its high mobility and also to the weak forces between H2 molecules. Enhanced, cost effective adsorbents are also needed for other important separations such as the capture of the greenhouse gas carbon dioxide from low pressure flue gas mixtures.
Energy efficiency, capital investment, and operating costs are all important factors to consider when putting a gas separation or storage technique into practice. Many absorbents suffer from low gas loading on a weight percentage basis. Energy is consumed during the adsorption/desorption cycle. Gas storage and separation using chemical reactions or clathrate-hydrate inclusion compounds are often energy intensive due to large heats of reaction or heats of formation. Likewise, regeneration of scrubbing solutions often requires significant quantities of heat.